Power line carrier telephone system



1953 R. c. EDSON POWER LINE CARRIER TELEPHONE SYSTEM Filed May 27, '1949 2 Sheets-Sheet l lNl/ENTOR RC. EOSON 0T. 0. W

ATTORNEY R. C. EDSON POWER LINE CARRIER TELEPHONE SYSTEM Oct. 6, 1953 2 Sheets-Sheet 2 Filed May 27, 1949 INVENTOR By RC. EDSON W 0. Cwfl.

ATTORNEY IRE Patented Oct. 6, 1953 UNITED STATES PATENT OFFICE POWER LINE CARRIER TELEPHONE SYSTEM Robert 0. Ethan; Livingston, ,N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New Yiirk Application May 27, 1949 SerialNo. ssgcss I 4 Claims. 1 i This invention relates to communication systems and particularly to .telephonesy'stems which include subscriber stations arranged for carrier current operation over a transmission line, which maybe either a power transmission line .or a line primarily constructed for communication purposes.

Objects of the invention are the provision of carrier telephone service over transmission lines to both carrier and non-carrier subscriber :stations served by the same power line or by interconnected power lines, and the provision of means whereby divided code ringing may be employed for ringing :both the carrier and non-carrier eta tions.

This invention is a telephone system comprising a .central oifice including subscriber lines, a power transmission line and carrier terminal equi ment interconnecting :one or more *of the subscriber lines with the power transmission line; whereby telephone service is provided for both carrier and non-carriersubscribers stations connected to the power line,

A feature of the invention is a telephone systern comprising a power transmission line in which divided code ringing is provided in the central oflice to signal both carrier and non-carrier stations connected to the power transmission line.

i -A- further feature is a telephone system com prising ai -carrier current transmission line for giving telephone service to a plurality of subscriber stations, the carrier terminal equipments at-the-ends' of said transmission line being connected to the same power supply and arranged for idii'rided code ringing.

A clear and complete explanation of the invention will be facilitated by a description of the system embodying the invention and its features, one such system employing a power transmission line as a carrier frequency transmission line being Shown schematically in the drawings accompa'nying this specification.

The drawings consist or two figures and schem-atically represent anautomatic telephone exchange having a plurality of subscriber stations connected thereto by-a power transmission line.

Referring to the drawings:

Fig. '1 shows adial telephone exchange which is represented by a subscribers station and line H, a connector switch Cl, a ringing code generator 21,- another subscriber line -2 I, a power transmission line PL, and a carrier terminal equipment CE-l interconnecting the line 2| and power line PL;- and Fig. 2 shows a carrier subscriber station H5 connected to the power line PL, and shows carrier-terminal equipment CEZ interconnecting the power line PL and a telephone line 2! to which subscribers stations MI and 2 13 are connected.

The subscribers stations 10,- 2 and 2l3are of the usual type provided for use on lines ter minating in an automatic central oflicewor exchange and each station includes a dial or other impulsesender (not shown) for use in selectively controlling the switches in the central oiiice through which desired connections are established. The station 2; includes a ringer 212 which is connected between one conductor of line 2N and ground; and the station 2L3 includes a ringer 214 which is connected between the other conductor of line 210 and ground. Other similar stations may be connected toyline 210 with ringers connected to the oneortheother of the line conductors. Although the ,line H is shown connected directly to a'connector switch GEL, it is understood that the connection therebetween ineludes other switches, :for instance, a line-finder switch and a firstselector switch. The connector Oil is represented by a :set of brushes and ,a single set of terminals, the associated relays and other equipment being omitted. "The line-finder, .selector and connector switches may be of the stepbyestep two motion type. Reference may be had to Automatic Telephony by .Smith and Campbell, 2d edition. pages 553 to '65, inclusive, for a detailed description of .the structure of step-:byestep switches and their operation when arranged for use as selectors and connectors. Reference may be had :to Patent 11,799.;654 granted to R. L. Stokely, April 7, .1931, ior a detailed disclosure of a subscriber line circuit and a line-finder of the step by-step type. The ringing code generator 21 is common to all of the connectors in the iofiice and comprises .a source LOf ringing current and means for interrupting the ringing supply conductors to produce a plurality of difierent ringingacodes for selectively signaling subscriber. stations. Reference may :be had to Patent 2 225 907 granted to J. Duguid, December 254, 1940,. for a disclosure of such a ringing code generator :and to Patent 2,289,503 granted .to C. D. .Koechling, July 14, 1942, fora detailed disclosure of a -.conn'ctor switch arranged for ten-party code ring- Thecarnier equipment CHM shown .in Fig; 1 :is similar to that shown in Fig. 2 of the patent application Serial No. :692,350 filed August :22, 1946, syn. C. mason-and L. Hochgraf, now'Patent 2.516.163 granted July 25, 1950; :the "carrier subscriber station 2l5 shown in Fig. 2 is similar tdthe carrier station shown in Fig. '3 "of the Edson l-Ioc'hgraf application; and the carrier equipment (3E2 shown in Fig. '2' is a modification of the carrier equipment shown in 3 or the same "patent application.

line 2|, ringing relays i and El, two relaxation oscillators OT and OR, relays M, 42, 44, 55 and 41 and a power transformer PTI together with other pieces of apparatus including filters, rectifiers, condensers and resistors. The middle left pair of windings 39 of the hybrid coil HC are connected by normally open contacts of a control relay 42 to the conductors of line 2I and the closure of a direct-current circuit through these windings in series with line 2| is under the control of relay 4?. The middle right pair of windings 33 are normally connected in series with the balancing network 34 to balance the line 2 I and a resistor 35 is provided for connection in place of the balancing network 34 on reverting calls, that is, on calls between stations connected to the same central ofiice subscriber line. The upper pair of windings 3! of the hybrid coil receive voice frequency currents from the carrier receiver CRIa, and the lower pair of windings 32 of the hybrid coil transmit voice frequency current from line ZI to the carrier transmitter CTI. The carrier receiver CRIa comprises a receiving filter IUI, tuned to receive voice modulated and signal modulated carrier currents of a frequency F2 over the power line from any of the subscriber telephone stations associated therewith, an amplifier I92 and demodulators E05 and I09. The carrier receiver CRlb comprises a receiving filter I01 tuned to receive carrier current of a frequency F3 transmitted over the power line on a reverting call from one of the subscriber stations, an amplifier I08 and a demodulator IE9.

The carrier transmitter CTI comprises an oscillator modulator tube 3!] and an amplifier tube 98. The oscillator tube has associated therewith the tuned circuit comprising inductance SI, capacitor 82, and a piezocrystal 84 connected in circuit to generate oscillations at a desired carrier frequency Fl. The tuned circuit has a high impedance at frequencies other than that of the crystal 84 so as to prevent oscillations at frequencies other than the desired carrier frequency which is determined by crystal 84. Since voice frequency current from telephone line H is transmitted to tube 80 through the lower pair of windings 32 of hybrid coil BC, the output current of tube BB includes both modulated carrier and voice frequency components. The network consisting of coil 95 and condenser 96 has a high impedance at the carrier frequency and a low impedance at voice frequencies; and element 94 has a low impedance at the carrier frequency and at voice frequencies. The voice modulated carrier frequency output of tube 80 is impressed through condenser 91 on the control grid of amplifier tube 98 and the amplified carrier current is transmitted through transformer 99, condenser We and coupler IIEI over the power line PL. The energy for operating tubes 80 and 98 and for operating the vacuum tubes of the carrier receivers CRI a and CR1?) is obtained from the power line through the transformer PTI. Other power distribution transformers may interconnect transformer PTI and the power line. Plate current for the vacuum tubes is supplied through part of winding 74 of transformer PTI, rectifier I5, filter 1'6 and conductor I1. Winding I4 also supplies alternating voltage to the upper elements of each of tubes 51 and B1 of the relaxation oscillators to control the transmission of ringing signals over the power line to the carrier subscribers stations as hereinafter further described. Winding ll of power transformer PTI in combination with rectifier l2 constitutes a source of negative potential for operating relays 52 and 44 and for biasing the grid of tube 93. The filaments of tubes and 98 are heated by energy transmitted through windings l9 and id of power transformer PTI, these windings being shown adjacent the tubes. Other windings of power transformer PTI supply the current for heating the filaments of vacuum tubes of the carrier receivers CRla and CRIb'. The carrier receivers are normally energized ready to receive signals but the bias normally provided for tubes 88 and S8 is such that there is normally substantially no current in the plate circuits of these tubes. When the transmission of carrier current of frequency FI is desired, the operation of one of relays 42, El and 6| short-circuits biasing resistor 89 to decrease the negative bias on the grid of tube 80; and the operation of relay 42 or the breakdown of either one of gas-filled tubes 51 and 61 decreases the negative bias of tube 98 to increase the output of the amplifier. The operation of the carrier terminal equipment CEI of Fig. 1 is hereinafter further described on calls to and from the subscriber stations connected to the power line.

The carrier terminal equipment CEZ shown in Fig. 2 comprises a carrier receiver CR2 and a carrier transmitter GT2 both of which are connected by conductors II2 through a coupler III to the power line PL. Energy for the receiver and transmitter is supplied from the power line through transformers PTZ and PT3. Other power distribution transformers may interconnect transformers PT2 and PT3 and the power line. A winding of the power transformer PTZ is connected in series with a rectifier 200 and resistor 202 to conductor 203 which supplies plate current for the carrier transmitter GT2 and carrier receiver CR2. The carrier transmitter GT2 comprises a modulator and amplifier and output filter FL! and two piezocrystals I52 and I53 connected with the oscillator modulator tube (not shown) to generate carrier current of frequency F2 or frequency F3 as desired, the operative connection of the one or the other of the crystals being controlled by a relay KI. The carrier receiver CR2 comprises filter units FLZ and FL3 which transmit current of carrier frequency FI and suppress components of other frequencies, an amplifier I25, an interstage filter FL4, an amplifier I32, an output filter FLB and a demodulator I61. The filters FL2, FL3 and FL l and amplifier I25 are similar to the like identified elements in the aforementioned E dson-Hochgraf patent application. The carrier terminal equipment further includes relay K2 connected in series with rectifier I4! for operation by incoming carrier current, transformers I65 and I66 for transmitting voice currents between the voice line 2H] and the carrier current transmitter GT2 and carrier current receiver CR2, a line relay I69, and a ringing control apparatus which includes relays R, T, RR, and RT, vacuum tube I90, and a number of resistors, condensers and rectifiers. Windings of power transformer PT3 supply ringing voltage for operating the ringers at stations 2, 2I3; another winding of transformer PT3 together with rectifiers I558 and I99 supplies the current for operating various relays and the control potentials for the grid elements of tube I; another winding ,(not shown) supplies the current for the filaments of tube I90.

Assume now that a call is initiated at station Ill and that the called station is one of the stations connected toline 2). The line H is extended by the operation of a line-finder 'switch (not shown) and by the operation of one or more selector switches (not shown) and connector CI responsive to the dialing of the called stations directory number to select the line 2| which connects to the carrier terminal equipment CEI; and, in response to the dialing of the party identifying digit of the called number, the connector CI selects the particular ringing code which is to be used to signal station 2| I. The connector C| tests the terminals to which line 2| is connected to determine whether or not the line is busy; and, if the line is idle, completes the ringing circuit whereby the required ringing code is transmitted over either the tip or the ring conductor of line 2| to operate the one or the other of ringing relays 5| or 6|. The ringing current frequency may be 20 cycles per second and relays 5| and 6| are of a type which is operatively responsive to currents of such a frequency. If the called'station is signaled over the tip conductor of line 2 the ringing current effects the operation of relay 5| of the carrier terminal equipment CEI during each ringing interval of the selected ringing code; and, if the called station is signaled over the ring conductor of line 2|, the ringing current effects theoper'ation of relay 6| of the carrier terminal equipment CEI during each ringing interval of the selected ringing code. The operation of either relay 5| or relay 6|, responsive to ringing current transmitted over line 2 I, closes a short circuit across resistor 89 thereby reducing the grid bias of oscillator tube 8|] of carrier transmitter CT I. The operation of relay 5| connects the plate supply conductor 11 to relaxation oscillator OT and the operation of relay 6| connects the plate supply conductor 11 to relaxation oscillator OR. The two relaxation oscillators OT and OR are made up of similar elements and operate in like manner. Assume that relay 5| is operated by the ringing current received over line 2| from connector CI. The operation of relay 5| closes a circuit for charging condenser 56, this circuit being traced from plate supply conductor 11, through a front contact of relay 5|, resistors52 and 55, condenser 58, resistor 5S, conductor 1|], winding 1| of power transformer PT| and rectifiers 12 to ground. The charging of condenser 56 builds up a voltage across the control gap of gas-filled tube 51, a predetermined interval of time being required to charge condenser 55 to a voltage suffi'cient to ionize the control gap of tube 51. As soon thereafter as the winding 14 of power transformer PT| is energized in a direction which renders the main anode of tube 51 positive with respect to ground potential, a discharge is started between the main anode and cathode of tube 51. The voltage drop through resistor 59 is thus increased so as to reduce thebias applied through resistors 58 and 39 to the grid of tube 98 and to reduce the bias applied through resistor 38 and windings 32 of hybrid coil HC to a grid of tube 85; whereby carrier current of frequency Fl is generated and transmitted through transformer 99 and coupler "I, over the power line to each of the carrier subscriber stations. As the potential of the cathode of tube 51 rises, the voltage applied through condenser 56 and resistor 55 to the gas-filled tube 53 is increased until the discharge occurs in gas-filled tube The potential of the starter anode of tube 51 is reduced due to discharge tube 55'. When the power voltage begins tofall, so also does the potential of the main anode of tube 51; and,

when this potential falls to a value which is insuflicient to maintain a discharge between the main anode and cathode of tube 51, the discharge in tube 51 ceases, tube 53 is deenergized and the potential applied to the grids of the transmitter tubes and 98 becomes sufficiently negative to terminate the transmission of carrier current of frequency Fl over the power line PL. The voltage between conductors 10 and 11, the capacity of condenser 56, and the resistance of element 55 are such that it takes a predetermined interval of time for the voltage across the control gap of tube 51 to reach the breakdown value. It is the function of tube 53 to stabilize the oscillator OT against variations in the supply voltages to the oscillator obtained from the power line. With tube 53 provided as shown, the voltage from which condenser 56 starts to recharge becomes lower as the supply voltage becomes higher; and the voltage from which condenser 56 starts to recharge becomes higher as the supply voltage becomes lower. sures a very nearly constant time for the volt age across the control gap of tube 51 to reach the breakdown value, During the interval which is measured by the charging of condenser 56, the winding 14 of power transformer PT| will have been energized by a half cycle of oppositepo'- larity and by another half cycle of power wave of the polarity which tends to make the main anode of tube 51 positive with respect to ground; but no impulse of carrier current of frequency Fl is transmitted during this period because at the time the anode of tube 51 becomes positive, the voltage across .the control gap has not reached a value sufiicient to effect breakdown. During the following half cycle of the power wave, while the main anode of tube 51 is negative with respect to ground, the voltage across condenser 56 increases sufliciently to effect ionization of the control gap; and as soon thereafter as the voltage induced in winding 14 once more makes the main anode of tube 51 sufliciently positive with respect to ground, the main gap of tube 51 is ionized and the transmitter CTI is energized to transmit another impulse of carrier current of frequency Fl. Thus, every second positive half cycle of the power wave effects the breakdown of tube 51 as long as relay 5| remains operated. Thus, assuming the power line fre-- quency to be 60 cycles per second, carrier current impulses of about 7 milliseconds duration are transmitted over the power line during each ringing interval of the ringing code transmitted from connector Cl over line 2| to relay 5|, the impulses being transmitted over the power line at a rate of 30 impulses per second.

If ringing current is received over the ring conductor of line 2| instead of the tip conductor, relay 6| is operated instead of relay 5|; and oscillator OR is energized in similar manner to that in which oscillator OT is energized as above described. The carrier transmitter CTI is thereby energized to transmit carrier current impulses of frequency Fl over power line PL at a rate of 30 impulses per second during every second half cycle of the voltage wave induced in winding 14 of power transformer of the polarityopposite to that which effects the operation of oscillator when relay 5| is operated, half cycles of theoth'er The provision of tube 53 thus in polarity of" the power wave cause'the operation of oscillator OR to send ringing signal impulses of carrier current over the power line.

In some cases, the central office may be arranged to transmit ringing current over one conductor of a line to ring the calling station at the same time that ringing current is being transmitted over the other conductor of the line to ring the called station. In such a case, both of relays i and 9! are'operated at the same time; and carrier current is transmitted during both the positive and negative halves of alternate cycles of the power voltage.

The ringing signal impulses of carrier current of frequency FI, generated by carrier current transmitter CTI responsive to ringing current transmitted over line 2! from the central ofliee to the carrier equipment CEI, are transmitted over the power line PL to each of the carrier subscriber stations such as 2I5 and also to the carrier terminal equipment 0E2. At the carrier subscriber station, the operations in response to the ringing signal are the same as described in the aforementioned Edson-Hochgraf patent application. The ringing signal carrier current impulses incoming through the coupler II I to carrier equipment 0E2 are transmitted through conductors I I2, filter FLZ and FL3 of the carrier receiver CR2, amplifier I25, and filter FLA to the amplifier I32. The positive potential supply conductor 203 is connected through resistor I36 to the screen grid of tube I32 and is further connected through the left winding of transformer I to the anode of tube I32, so that tube I32 is effective to amplify ringing signal carrier current impulses received during both positive and negative half cycles of the power wave. The right winding of transformer I45 is connected in series with rectifier I47, retard coil I49, conductor I and the winding of relay K2; whereby relay K2 operates and releases at a 30-cycle rate in response to the rectified impulses of carrier current of frequency FI whenever ringing current signal impulses of this frequency are received from the power line. Each operation of relay K2 closes a circuit from positive potential supply conductor 203, through a back contact of relay R, resistor I83, front contact of relay K2, winding of relay KI, a back contact of relay C, resistor I13 and a back contact of relay T to ground. The winding of relay KI is shunted by condenser I54 which charges when relay K2 operates and which discharges through resistor I55 each time relay K2 releases. The capacity of condenser I54 and resistance of element I55 have values required to prevent the operative energization of the winding of relay KI responsive to the alternate operation and release of relay K2 at the rate of 30 cycles per second. At the same time that relay K2 is operatively responding to the rectified ringing signal impulses, these direct-current impulses are being applied through conductor I50 and back contacts of relays KI and C, through a transient eliminating network consisting of resistor I9I, condensers I92 and I93 and varistor I94, and through resistors Ill and I8? to the grids of the double triode tube I90. The grids of tube I90 are normally negatively biased to cut off thereby making both sections of the tube responsive only to the application of positive voltage to the grid. The biasing circuit is traced from ground through the one or the other of rectifiers I98 and I99 and the associated half of winding 205 of transformer PT3, resistor I91 and resistor I95, whereby the drop across resistor I99 is applied through re- Ilw 8 sistor I95 to the grid of each triode. If the voltage applied through conductor I50 consists of a single transient, resulting from other operating functions, the above network will effectively reduce the positive voltage from this transient to prevent the operation of either of relays T and R. Normally condenser I92 is in a discharged state due to resistor I9I. When a transient of positive voltage appears on conductor I50, condenser I92 will be charged through varistor I94 and the voltage at the junction of varistor I94 and condenser I93 will not build up to a value sufiicient to make either section of vacuum tube I90 operative. After the transient voltage has ceased, condenser I92 will discharge through resistor I9I. However, when the voltage on conductor I50 consists of rectified 30-cycle positive ringing pulses, a charge will build up to remain on condenser I92 due to the time constant of the circuit; and this charge will maintain varistor I90 in a high impedance state so that the 30- cycle positive pulses will be transmitted through condenser I93 and make one or the other of the triodes of tube I90 operative, depending upon which plate element is positive at the same time the corresponding grid has been made less negative by the positive pulse.

The plate of the left-hand triode is connected through the winding of relay R, resistor I85 and winding 208 of transformer PT3; and the plate of the right-hand triode is connected through the winding of relay T, resistor I15 and winding 291 of transformer PT3. Since the voltages induced in windings 201 and 208 are 180 degrees out of phase, the plates of the two triodes will "alternately have impressed thereon a positive potential with respect to ground. Assuming that the power line voltage applied to the carrier terminal equipment CEI is in phase with the power line voltage applied to carrier terminal equipment (3E2, positive potential pulses applied to the grids of tube I will be synchronized with the positive potentials applied to the plates, so that relay R is operated by breakdown of the left triode of tube I90 when the carrier terminal equipment CEI transmits ringing signal carrier current impulses responsive to the operation of relay GI; and relay T is operated by breakdown of the right triode of tube I90 when the carrier terminal equipment CEI transmits ringing signal carrier current impulses responsive to the operation of relay 5i. The condensers connected in parallel with the windings of relays T and R delay their operative energization to further reduce the likelihood of false operation responsive to a single transient and smooths out the 30- cycle impulse waves so as to hold the relay operated responsive to the ringing signal. The operation of either of relays T and R opens the above traced circuit path through the winding of relay KI to prevent its operation during the remainder of a ringing interval. If relay T is operated, it closes a circuit for operating relay RT and if relay R is operated, it closes a circuit for operating relay RR. The operation of relay RT disconnects the tip conductor of line 2 I 0 from hybrid coil I56 and connects this conductor through ballast lamp 209 and windings 206 and 201 of transformer PT3 to ground, thereby effect- .ing the transmission of ringing current over the tip conductor of line 2I0 to actuate the ringer 2I2 of station 2 and the ringers of any other stations which are signaled over the tip conductor of line 2I0. The operation of relay RR disconnects the ring conductor of line 2I0 from conductor of line 2I0 to actuate ringer 2I4 of station 2I3 and the ringers of any other stations which are signaled over the ring conductor of line 2 I0. Varistors I12 and I82, individually connected in parallel with the windings of relays RR and RT, are poled to make these relays slow to release and thereby prevent shortening of the ringing interval due to delays in operating relays K2, R and T.

Since the phase of the ringing signal voltages applied to the grids of tube I90 depends upon the phase of the power supply voltage at carrier terminal CEI and the phase of the voltages ap plied to the plates of tube I90 depends on the phase of the power supply voltage at carrier terminal E2, it may be necessary to change the phase of the voltages applied to the plates to correct for any phase shift of the power supply voltages between the carrier terminals. In this case, a phase shifting network consisting of resistors I15 and I85 and condensers I16 and I86 is provided to obtain the required phase relationship, the values of these resistors and condensers depending upon the phase shift required. If the phase relationship is satisfactory without any phase correction, the network is omitted.

When the receiver is removed at th called. stationv to answer the call, a circuit is closed during a silent interval of the ringing code for operating line relay I60, this circuit being traced from direct-current potential supply conductor 204, re-

sistor I62, top winding of relay I69, bottom windings of hybrid coils I65 and I66, left back contacts of relay RT and RR, ring conductor of line are, through the .subscrihers subset, tip conductor of line-2 I0, right back contacts'of relays RT and RR, middle windings of hybrid coils. I66 and I 65, middle winding of relay I60 and through resistor I al to ground. The lower winding of relay I50 isa normally energized biasing winding. The operation of relay I60 closes a connection between the positive voltage supply conductor 203 and the plates of the oscillator-modulator and amplifier tubes (not shown) of the carrier transmitter (3T2. The operation ofrelay I60 also closes a circuit including resistor I54 for operatively energizing relay B. The operation of relay B closes a circuit including conductor 204 for operatively energizing the winding of relay C. The operation of relay 0 opens the normally closed short circuit across the left Windingof hybrid coil I65, opens the operating circuit of relay KI, opens the connection betweenconductor I50 and the grids of tube I90, closes the filament circuit for the amplifier tube of the transmitter GT2, and. connects ground to. conductor I56 to render the oscillatormodulator tube effective to generate and transmit carrier current of frequency F2 at a desired energy level. With relay KI normal and frequency control crystal I52 operatively connected to the oscillator of carrier transmitter CTZ carri'er current of frequency F2 is transmitted through filter FIJI, conductors H2, coupler ill and power line PL to the carrier terminal eq.uip ment- CEI and to each of the carrier subscriber stations 2 I-5 which are connected to power line PL.

Thecarrier current of frequency F2 transmitted from: carrier terminal equipment 0E2 when an incoming call? is answered at one of the subscriber stations: connected to line 2 i0 does not affect the carrier receiver at carrier subscriber station 2I5 since this receiver is tuned to a band including frequency Fl but excluding frequencies F2 and F3. Since the receiving filter l0I of carrier receiver CRIa at the carrier terminal CE! is tuned to pass a band including frequency F2, the carrier current of this frequency transmitted from the answering carrier station passes through coupler H6 and receiving filter Il I of carrier receiver CRIa of the carrier equipment CElI, to the input circuit of amplifier I02. Rectifier element Silt transmits a part of the output of amplifier I82 to effect the energization of the windings of relays i! and 4| in series; and rectifier element I05 transmits voice frequency components in the output circuit of amplifier I02 through condenser 49 and windings 3| of hybrid coil H0. The operation of relays 41 and 4| by the rectified carrier current of frequency F2 is delayed for a predetermined interval of time, While condenser 43 is charging to prevent operation of these relays by static or other momentary disturbances on line PL. The operation of relay 4| opens a normally closed short circuit across relay 45 and closes a circuit for operating relay 42. The operation of relay 42 connects the con ductors of line 2! to windings 30 of hybrid coil HC; and with relay 4'! also operated, thesewindings are connected in series across the conductors of line M to cause the operation of the ringing trip relay (not shown) of the connector CI and the closing of the talking path through the connector between the calling station I0 and line 2I. The aforementioned operation of relay 42 closes a short circuit across resistor 89 to reduce the rid bias of tube and closes a short circuit across resistors 38 and 35 to reduce th grid bias of tube 58 so that carrier current of frequency FI is transmitted through transformer 39 and coupler I I0 over power line PL. The carrier current of frequency FI thus transmittedover the power line effects the continuous operation of relay K2 of carrier equipment 0E2. Voi'ce currents transmitted from the called subscriber station over line 2i!) through the windings of hybrid coils I65 and IE6 modulate the carrier current of frequency F2 transmitted by the carrier transmitter GT2 over the power line PL to the carrier receiver CRIa and thence through rectifier I05 and the windings 3! and 30 of hybrid coil H'C, through line 2I- and th talking connection inthe central offic'e to the calling subscriber station I0. Voice currents incoming over line 2I from the calling station 0 are impressed through windings 30 and 32 of hybrid coil HC on a grid of tube 80 thereby to modulate the carrier current of frequency FI- being transmitted over the power line; and these voice current components of the carrier current of frequency FI incoming to carrier receiver CR2 are transmitted through conductor I50, condenser I5I, and windings of hybrid coils I65 and IE6 over line 2 I0 to the called subscriber station. Thus two-way Voice transmission between station I0 and the called carrier station is effected. and the connection is maintained until released at the calling and called stations. When the handset is replaced on the hook at the called carrier subscriber station, relay I60 releases thereby deenergizing' carrier transmitter GT2 and releasing relays B and C; but relay K2 of carrier equipment 0E2 remains operated until carrier transmitter CTI of the carrier terminal equipment CEI' is also deenergized. When the trans mission of carrier current of frequency F2 from carrier transmitter GT2 ceases, relays 47 and M of the carrier terminal equipment CEI release.

The release of relay 4? opens the bridge across the conductors of line 2| and the release of relay 4| causes the release of relay 42. The releas of relay 42 reconnects line 2| to ringing relays iii and 6|, opens the short circuit across biasing resistor 89, and opens the short circuit across biasing resistors 38 and 36 whereby the transmission of carrier current of frequency Fl from transmitter CT| is ended and relay K2 of carrier equipment CE2 and the corresponding relay at each of the carrier subscriber stations 2 are released. The line-finder LFl and selector Si are restored to normal in usual and well-known manner when the receiver is replaced at the callin station l6.

Call from carrier subscriber station to station 10 Assume now that the receiver is removed from the receiver hook at one of the subscriber stations connected to line 2 Hi to initiate a call to station l0. Relay I60 is thereby operatively energized, and carrier transmitter GT2 is energized in the manner above described to transmit carrier current of frequency F2 over the power line. This carrier current does not afiect carrier subscriber station 2|5 because the input filter of the carrier receiver thereat is tuned to pass a band which does not include frequency F2. At the carrier terminal equipment CE|, the carrier current of frequency F2 passes through filter [M of carrier receiver CRla and effects the operation of relays 4! and 4|. The operation of relay 4| causes the operation of relay 22; and with both of relays 41 and 42 operated, the windings 3B of hybrid coil I-IC are connected in series with the conductors of line 2| to cause the operation of the line relay (not shown) of.line circuit 22, and a line-finder LF2 is thereupon operated to extend the calling line 2| to a first selector S2. The aforementioned operation of relay 42 also reduced the bias of tubes 80 and 98 in the manner hereinbefore described to cause carrier transmitter CTI to transmit carrier current of frequency F| over power line PL. This carrier current is modulated by dial tone transmitted from selector S2 through line-finder LF2, line 2| and windings 30 and 32 of hybrid coil EC to the input circuit of tube 88, The dial tone modulated carrier current of frequency Fl thus transmitted over power line PL and incoming to carrier equipment CE2, is received by carrier receiver CR2; and the dial tone is transmitted through conductor I50, windings of hybrid coils I65 and I66 and line 2|!) to the calling station. When the calling subscriber hears the dial tone, the directory number of the called station I0 is dialed. The line relay E 58 is alternately released and reoperated under the control of the dial and the contacts of relay H50 open and close the connection between plate supply conductor 283 and amplifier-modulator of carrier transmitter CT2 to transmit over the power line a train of dial impulses corresponding to each digit dialed, each dial impulse being transmitted as an interruption of the carrier current of frequency F2. With relay 42 of carrier terminal equipment CEI operated, condenser 43 is connected in parallel with the winding of relay 4| alone, so that relay 4! becomes fast in operating and releasing and relay 4| becomes slow in releasing; so that relay 4| remains operated during the receipt of dial impulses but relay 2? is released and reoperated responsive to each impulse. Each release of relay 4! opens the bridge across the conductors of line 2|. The first train of dial impulses is thus repeated over line 2| to effect the selective operation of selector S2. The connection is extended by the operation of selector S2 to an idle connector C2 having access to the called line. The remaining digits dialed by the calling subscriber effect the selective operation of connector C2 to connect with the called line and to select the ringing code with which station I9 is signaled. The connector C2 makes the usual busy test and if line is idle, the selected ringing code is transmitted over line I to signal the called subscriber. When the called subscriber answers, the ringing trip relay (not shown) of connector C2 is operated to terminate the ringing and the talking connection is established between lines 2| and II. Voice currents transmitted from the calling station over line 2|B modulate the carrier current of frequency F2 transmitted over the power line PL from the carrier transmitter GT2; and the carrier receiver CRia of carrier equipment CEI transmits these voice currents through demodulator |05, condenser 49, windings 3| and 39 of hybrid coil HC, over line 2|, through linefinder LF2, selector S2, connector C2 and line H to the called station it]. Voice currents from station ID are transmitted over line H, through connector C2, selector S2, line-finder LF2, line 2 I, and windings 3! and 32 of hybrid coil HC to modulate the carrier current of frequency F| being transmitted from carrier transmitter CT| over power line PL; and the carrier receiver CR2 at the calling carrier subscriber station demodulates this carrier current and transmits the voice currents through rectifier |4'|, conductor I50, condenser Isl, windings of hybrid coils N and I66 and line 2 Hi to the calling station.

The connection is maintained until the receiver is replaced on the receiver hook at the calling station, at which time the line relay I releases. The release of relay |!60 deenergizes the carrier transmitter GT2, causing the release of relay 41, ll and 42 of carrier terminal equipment CEI. The release of relay 4! opens the bridge across the conductors of line 2| whereby the connector 02, selector S2 and line-finder LF2 are restored to normal in usual and well-known manner. The release of relay 42 deenergizes the carrier transmitter CT| to stop the transmission of carrier current of frequency Fl over line PL; and relay K2 of carrier equipment CE2 then releases.

Reverting calls A call from one of the subscriber stations served by central o-flice line 2| to another of the stations served by the same line 2| is known as a reverting call and is initiated in the same manner as any other call. The operation of a carrier subscriber station 2|5 on a reverting call originated thereat is described in the aforementioned Edson- Hochgraf application. Nhen a reverting call is initiated at one of stations 2H and 213, a line finder LF2 is operated to connect line 2| to an idle selector S2; and the operation of the dial at the calling station effects the operation of the selector and a connector C2 to connect with the called line. The connector C2 is arranged to make a reverting call test in case the called line is found busy when connection is made therewith. A connector arranged to make a reverting call test is disclosed in the patent to H. Hovland, No. 1,849,087 granted March 15, 1932. Busy tone is transmitted from the connector C2 to the calling station and the subscriber thereat thereupon restores the receiver to normal to permit ringing of the called station. When the receiver is restored to normal at the calling station, carrier transmitter GT2. is deenergized and the transmission of carrier current of frequency F2 over line PL is terminated, thereby causing the release of relays 41., 4 l and of carrier terminal CEI. The bridge across conductors ofline 2! is opened by the release of relay 4! and the connector C2 then makes. the; revertive busy test above mentioned. Since the call being described; is a reverting call, the connector find-s thatthe called central office line; 2 I; is also the calling line; and the line-finder LE: and selector; S2. are thereupon restored to normal.v The connector C2 is held, however, with its brushes in, engagement with the terminals to which line. 21. is connected and ringing current is. transmitted according to the selected ringing code, over either the tip or the ring conductor of line 2t depending on whether the called station is atip station or a ring station The one or the other of ringing relay 51 and, 61. is operated by the ringing current to effect the transmission of impulses of carrier current of frequenc Fl from carrier transmitter CTI. over the power line PL to. actuate the ringers at all of the; tip stations served bycentral; Qfilce line 21., if the called stationis. a tip stationyor to. actuate the ringers at all ofthe ring stations served by line; 21!, if the calledstatiou is a. ring, station. If the callin station is not signaled 0. .1 the same side. of the line 2]., as is. the called station,. a short impulse of ringing. currenti-s transmitted at the end of each n ing c de ycle to effect av short peration of the: ringer at the calling. station so that the calling. par y will ltnow; when ringing ceases, that t e cal-1. as b en answ r d.

Assume first that the: called station is the carrier subscriber stationy zii. When the receiver is removed from the, receiver hook at the called station; to. answer the call, the carrier trans:- m-itter at station; 215 will be. energized to transmit. carrier current of frequency E2 over the power line PL, thereby operating relays 4'! and M of thecarrier terminal equipment CE'I. Relay 4| operates relay 42 and with relays 41 and 42 both operated, windings: 30' of hybrid coil HC are bridged acros the conductors. of line 2| to effect. the operation of the ringing trip, relay (not shown). of connector C2. The transmission of ringing current is, thereby. stopped and the brushes of connector C2 remain in engagement with the terminals of line 2 las long as thebridge is maintained across the conductors. of line. 21.. With relay 42 operated, carrier transmitter Cfll is energized to transmit. carrier current of frequency Fl, thereby operating the carrier equip.- ment. at. each carrier subscriber statioucQnnected to power line 13L, as. described in the aforementioned Edscneliochgraf application and. also. op.-. erating r lay K2 of car ier t rm nal equi m nt Cliil. associated with the calling. stationon line 2H). With. relay K2. continuously operated, relay- K l. is. also operatively energized; and relay Kl locks. independent ofrelay C aslong as relay K2 remains operated. When. the. subscriber at the calling station realizes the ringer is, no. longer being, actuated, the. receiver is, remoyedfor conversation with. the subscriber. at. the called. station. Relay I50. is thereby operated, in turn causing the operation. of relay B. followed by the operation of relay C. With, the relay Kl operatr ed, crystal I53, is included in. the frequency determinin circuit of: the oscillator t be o rans.- mitter GT2; and energization of the carrier trans,- mitter by the actuation of relay L6H causes the transmission, oi carrier current of frequency F3 ov r. th po r. line P The arrier. current. o

of relay 42'.

frequency F3 thus transmitted over line PL, is without effect at any of the carrier subscriber stations; but at the carrier terminal CEI, this current passes through filter I01 of carrier receiver CRIB to amplifier I08. The current in the output circuit of amplifier I08 is demodulated by rectifier I09; and since the short circuit across the winding of relay 45 has been opened. by the operation of relay 4| responsive to carrier current of frequency F2 received over line PL from the called carrier subscriber station, relay 45 is operatively energized. Relay- 45 closes a circuit for operating relay 44. The operation of relay 44 opens the operating circuit of relay 45 and con nects the output of demodulator H19 to windings 3| of hybrid coil HC. Conversation between the calling and called stations is thus enabled, voice currents from the telephone transmitted at the called station being effective to modulate the carrier current of frequency F2 transmitted from the associated carrier transmitter over line PL to carrier receiver CRla of the carrier terminal CE! The voice current components of the carrier current of frequency F2 received from the called station are transmitted through demodulator I05, condenser 49 and windings 3| and 32 of hybrid coil HG to modulate the carrier current of frequency Fl transmitted from carrier transmitter CT! over line PL to both the calling and called carrier subscriber stations. The voice currents from the telephone transmitter at the calling station modulate the carrier current of frequency F3 transmitted from the associated carrier transmitter GT2 over line PL to carrier receiver CRlb of the carrier terminal. CT. The voice current components of the carrier current of frequency F3 received from the calling station are transmitted through demodulator I09, windings 3i and 32 of hybrid coil HCto modulate the carrier current of frequency Fl transmitted from carrier transmitter CTI over line PL to both the calling and called carrier subscriber stations.

If the calling subscriber restores. the telephone setto normal at thesame time or before the. telephone set is restored at the called station, the associated carrier transmitter GT2 is deenergized to terminate the transmission of carrier current of frequency F3, relay 45 of carrier equipment QEI being thereby released. When the carrier transmitter at the called carrier subscriber station 215 is also deenergized, the transmission of carrier current of frequency F2 is terminated thereby causing the release. of relays 41, 4| and d2 of carrier terminal CE]. The release, of relay 42. deenergizes carrier transmitter CTI thereby causing the release. of the operated relays at each of the. carrier subscriber stations and the release of relays K2 and Kl of equipment 0E2. The release of relay- 4! opens the bridge across line 21 thereby causingthe. connector C2 to be restored to normal in usual manner.

If the handset is, restored to. normal at the called carrier subscriber station 215. before the connection is released at the calling station, the deenergization ofythe carrier transmitter at the called station causes the release of relays 41 and (if of carrier terminal CEf'l. The release of relay 4"! opens the bridge across the conductors. of line 21' and the. release of relay ll causes the release The release of relay '42 restores. the normal grid bias of the carrier transmitter tube to terminate the transmission of carrier current of frequency Fl' over line PL. and thereby causes the release of' the operated relays. at each of the carrier subscriber stations and the release of relays K2 and Ki of equipment 0E2. The release of relay Kl of the equipment 0E2 changes the tuning of carrier transmitter GT2 by inserting crystal I52 in place of crystal I53 in the frequency determining circuit of the oscillator; so that the carrier current transmitted therefrom is changed from frequency F3 to frequency F2. lhis change in the frequency of the carrier circuit transmitted over line PL from the carrier equipment 0E2 causes the release of relays 45 and 3 and the reoperation of relays ill, ll and 42 of carrier terminal GEL The reoperation of relays H and 42 again closes the bridge across the conductors of line 25. If the interval during which relays M and 42 were released is sufficiently long, the connector switch C2 has at that time been restored to normal; and in such a case the reclcsing of the bridge effects the operation of the line relay of line it and the starting of a line-finder. The reoperation of relay 12 again energizes carrier transmitter CT! so that carrier current of fre quency Fl is again transmitted over line PL, operating relays at all carrier subscriber stations and reoperating relay K2 of carrier equipment 0E2. Since the circuit for operating relay Kl is open at a contact of relay C, relay Kl does not reopenate and carrier current of frequency F2 is transmitted until the telephone is replaced at the calling station. When the receiver is restored to normal at the calling station on line 2 l8, relay lfiil releases. The release of relay 9% causes the successive release of relays B and C and deenergizes the carrier transmitter 0T2 to stop the transmis sion of carrier current of frequency F2 and thereby effect the release of relays 41, M and 32 at the carrier terminal CEI. The release of relay l? again opens the bridge across the conductors of line H thereby to effect the return of connector C2 to normal, if not already returned to normal; or if connector C2 was returned to normal before the carrier transmitter at the calling station started to transmit carrier current of frequency F2, the opening of the bridge across the conductors of line 21 causes the release of the linefinder which was operated to connect with line 2! after the connector C2 was released.

If both the calling and called stations are connected to line are, the operations of the carrier equipment 0E2 are the same as above described, as though a single one of the stations on line Eli] were involved on the call. The voice frequency currents are transmitted over that part of line 2H! which interconnects the two stations. Another transmission path between these stations includes the carrier frequency equipments CB2 and CE! and power line PL, in order to maintain the connector C2 in operated position in engagement with the terminals of line 2| until the conversation is terminated. The transmission delay in the carrier path will not be sufficient to produce appreciable distortion of the transmission between the two stations.

What is claimed is:

1. In combination in a power line carrier communication system, an alternating-current power line, a carrier terminal unit connected to a particular phase of said power line, a plurality of telephone stations, a two-conductor voice frequency line connecting said stations to said carrier terminal unit, said carrier terminal unit comprising means for demodulating carrier current incoming over said power line, first control means connected to said demodulating means for operation when carrier current is being received over said power line during half cycles of one polarity of said phase of the power line voltage, second control means connected to said demodulating means for operation when carrier current is being received over said power line during half cycles of the opposite polarity of said phase of the power line voltage, means including transformer means connected to said particular phase of said power line supplying operating potential to said first control means during half cycles of one polarity of said phase of the power line voltage and supplying operating potential to said second control means during half cycles of the opposite polarity of said phase of the power line voltage, a ringing current source, relay means connected to and operated by said first control means for connecting said source to one of the conductors of said voice frequency line to signal some of said stations, and relay means connected to and operated by said second control means for connecting said source to the other of the conductors of said voice frequency line to signal the remainder of said stations.

2. In a telephone system comprising subscriber lines including multistation lines, a first ringing current source, means for selectively connecting said source to the one or the other conductor of a subscriber line to transmit a desired one of a plurality of different ringing codes over the selected line conductor, an alternating-current power line, a first carrier terminal unit connecting one of said multistation lines to said power line, a two-conductor voice frequency line, a plurality of subscriber stations connected to said voice frequency line, a second carrier terminal unit interconnecting said power line and said voice frequency line, transformer means supplying voltage of the same phase from said power line to both of said carrier terminal units, transmitting means in said first unit for transmitting carrier current over said power line to said second unit, a first signal means in said first unit connected for operation by ringin current from said source incoming over one of the conductors of said multistation line, a second signal means in said first unit connected for operation by ringing current from said source incoming over the other conductor of said multistation line, first control means including windings of said transformer means and said first signal means connected to energize said transmitting means to transmit carrier current over said power line during half cycles of one polarity of said phase of the power line voltage When said first signal means is operated by ringing current from said source incoming over said one conductor of the multistation line, second control means including windings of said transformer means and said second signal means connected to energize said transmitting means to transmit carrier current over said power line during half cycles of the other polarity of said phase of the power line voltage when said second signal means is operated by ringing current incoming over said other conductor of the multistation line, demodulating means in said second carrier terminal unit for demodulating carrier current incoming thereto over the power line, a second ringing current source, a first relay means for connecting said second source to one of the conductors of said voice frequency line to signal some of said stations connected thereto, a second relay means for connecting said second source to the other of the conductors of said voice frequency line to signal the remainder of said stations, first control means connected to said demodulating means for operating said first relay means when carrier current is being received over said power line during half cycles of one polarity of said phase of the power line voltage, second control means connected to said demodulating means for operating said second relay means when carrier current is being received over said power line during half cycles of the opposite polarity of said phase of the power line voltage, and means comprising windings of said transformer means supplying operating potential to said first control means during half cycles of one polarity of the power line voltage and supplying operating potential to said second control means during half cycles of the opposite polarity of the power line voltage.

3. In a telephone system comprising subscriber lines including multistation lines, a first ringing current source, means for selectively connecting said source to the one or the other conductor of a subscriber line to transmit ringing current in a desired one of a plurality of different ringing codes over the selected line conductor, an alternating-current power line, a first carrier terminal unit connecting said power line to one of said multistation lines, a two-conductor voice frequency line, a plurality of subscriber stations connected to said voice frequency line, a second carrier terminal unit connecting said power line to said voice frequency line, transformer means supplying voltage of the same phase from said power line to both of said carrier terminal units, carrier current transmitting means in said first unit for transmitting carrier current over said power line to said second unit, first signal means in said first unit connected for operation by ringing current incoming over one conductor of said one line, second signal means in said first unit connected for operation by ringing current incoming over the other conductor of said one line, control means in said first carrier terminal unit including said first and second signal means and windings of said transformer windings connected to energize said transmitting means to transmit carrier current over said power line during half cycles of one polarity of said phase of the power line voltage when said first signal means is operated and to energize said transmitting means to transmit carrier current over said power line during half cycles of the opposite polarity of said phase of the power line voltage when said second signal means is operated, demodulating means in said second carrier terminal unit connected for demodulating carrier current incoming over said power line, a second ringing current source, control means in said second unit comprising two triodes and windings of said transformer means for selectively effecting connection of said second source to the one or the other of the conductors of said voice frequency line, means connected to said demodulating means for applying a positive potential to the control elements of said triodes when carrier current is being received by said demodulating means, the cathodes of said triodes being connected to the midpoint of a secondary winding of said transformer windings and the anodes of said triodes being connected respectively to the opposite ends of said secondary winding whereby one of said triodes is energized by application of said positive potential to its control element during half cycles of one polarity of said phase of the power line voltage and the other of said triodes is energized by application of said positive potential to its control element during half cycles of the opposite polarity of said phase of the power line voltage, means comprising a relay connected in the anode-cathode circuit of one of said triodes for connecting said second source to one conductor of said voice frequency line to signal the station having ringers connected to said one conductor, and means comprising a 'relay connected in the anode-cathode circuit of the other of said triodes for connecting said second source to the other conductor of said voice frequency line to signal the stations having ringers connected to said other conductor.

4. In a telephone system, a carrier frequency transmission line, a first carrier terminal unit connected to said line, a second carrier terminal unit, remote from said first carrier terminal unit, connected to said line, interconnected power lines and transformer means supplying voltage of the same phase to both carrier terminal units, a twoconductor voice frequency line connected to said second carrier terminal unit, a plurality of subscriber stations connected to said voice frequency line, transmitting means in said first carrier terminal unit for transmitting carrier current over said carrier frequency line, means comprisin windings of said transformer means selectively applying an energizing potential to said transmitting means during half cycles or" one polarity of said phase of the power line voltage or during half cycles of the opposite polarity of the said phase of the power line voltage, demodulating means in said second carrier terminal unit for demodulating carrier current incoming over said carrier frequency line, a ringing current source, first control means connected to said demodulating means for connecting said source to one of the conductors of said voice frequency line to signal certain of the stations connected thereto when carrier current is received by said demodulating means during half cycles of one polarity of said phase of the power line voltage, second control means connected to said demodulating means for connecting said source to the other of the conductors of said voice frequency line to signal the other of the stations connected thereto when carrier current is received by said demodulating means during half cycles of the opposite polarity of said phase of the power line voltage, and means including windings of said transformer means supplying operating potential to said first control means during half cycles of one polarity of said phase of the power line voltage and supplying operating potential to said second control means during half cycles of the opposite polarity of said phase of the power line voltage, whereby said first control means is operated when carrier current is being received by said demodulating means during half cycles of one polarity of said phase of the power line voltage and said second control means is operated when carrier current is being received by said demodulating means during half cycles of the opposite polarity of said phase of the power line voltage.

ROBERT C. EDSON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,289,048 Sandalls July 7, 1942 2,294,905 Honaman Sept. 8, 1942 2,378,326 Rees June 12, 1945 2,430,471 Lang Nov. 11, 1947 2,440,239 Almquist Apr. 27, 1948 

